The present invention relates to body implantable medical devices, and more particularly to stents and other prostheses configured for high radio-opacity as well as favorable mechanical characteristics.
Recently several prostheses, typically of lattice work or open frame construction, have been developed for a variety of medical applications, e.g. intravascular stents for treating stenosis, prostheses for maintaining openings in the urinary tracts, biliary prostheses, esophageal stents, renal stents, and vena cava filters to counter thrombosis. One particularly well accepted device is a self-expanding mesh stent disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The stent is a flexible tubular braided structure formed of helically wound thread elements. The thread elements can be constructed of a biocompatible plastic or metal, e.g. certain stainless steels, polypropylene, polyesters and polyurethanes.
Alternatively, stents and other prostheses can be expandable by plastic deformation, usually by expanding a dilation balloon surrounded by the prosthesis. For example, U.S. Pat. No. 4,733,665 (Palmaz) discloses an intraluminal graft constructed of stainless steel strands, either woven or welded at their intersections with silver. U.S. Pat. No. 4,886,062 (Wiktor) features a balloon expandable stent constructed of stainless steel, a copper alloy, titanium, or gold.
Regardless of whether the prosthesis is self-expanding or plastically expanded, accurate placement of the prosthesis is critical to its effective performance. Accordingly, there is a need to visually perceive the prosthesis as it is being placed within a blood vessel or other body cavity. Further, it is advantageous and sometimes necessary to visually locate and inspect a previously deployed prosthesis.
Fluoroscopy is the prevailing technique for such visualization, and it requires radio-opacity in the materials to be imaged. The preferred structural materials for prosthesis construction, e.g. stainless steels and cobalt-based alloys, are not highly radiopaque. Consequently, prostheses constructed of these materials do not lend themselves well to fluoroscopic imaging.
Several techniques have been proposed, in apparent recognition of this difficulty. For example, U.S. Pat. No. 4,681,110 (Wiktor) discloses a self-expanding blood vessel liner formed of woven plastic strands, radially compressed for delivery within a tube. A metal ring around the tube is radiopaque. Similarly, U.S. Pat. No. 4,830,003 (Wolff) discusses confining a radially self-expanding stent within a delivery tube, and providing radiopaque markers on the delivery tube. This approach facilitates imaging only during deployment and initial placement.
To permit fluoroscopic imaging after placement, the stent itself must be radiopaque. The Wolff patent suggests that the stent can be formed of platinum or a platinum-iridium alloy for substantially greater radio-opacity. Such stent, however, lacks the required elasticity, and would exhibit poor resistance to fatigue. The Wiktor ""110 patent teaches the attachment of metal staples to its blood vessel liner, to enhance radio-opacity. However, for many applications (e.g. in blood vessels), the stent is so small that such staples either would be too small to provide useful fluoroscopic imaging, or would adversely affect the efficiency and safety of deploying the stent or other prosthesis. This Wiktor patent also suggests infusing its plastic strands with a suitable filler, e.g. gold or barium sulfate, to enhance radio-opacity. Wiktor provides no teaching as to how this might be done. Further, given the small size of prostheses intended for blood vessel placement, this technique is unlikely to materially enhance radio-opacity, due to an insufficient amount and density of the gold or barium sulfate.
Therefore, it is an object of the present invention to provide a stent or other prosthesis with substantially enhanced radio-opacity, without any substantial reduction in the favorable mechanical properties of the prosthesis.
Another object is to provide a resilient body insertable composite filament having a high degree of radio-opacity and favorable structural characteristics, even for stents employing relatively small diameter filaments.
A further object is to provide a process for manufacturing a composite filament consisting essentially of a structural material for imparting desired mechanical characteristics, in combination with a radiopaque material to substantially enhance fluoroscopic imaging of the filament.
Yet another object is to provide a case composite prosthesis in which a highly radiopaque material and a structural material cooperate to provide mechanical stability and enhanced fluoroscopic imaging, and further are selectively matched for compatibility as to their crystalline structure, coefficients of thermal expansion, and annealing temperatures.
To achieve these and other objects, there is provided a process for manufacturing a resilient body insertable composite filament. The process includes the following steps:
a. providing an elongate cylindrical core substantially uniform in lateral cross-section and having a core diameter, and an elongate tubular case or shell substantially uniform in lateral cross-section and having a case inside diameter, wherein one of the core and case is formed of a radiopaque material and the other is formed of a resilient material having a yield strength (0.2% offset) of at least 100,000 psi, wherein the core diameter is less than the interior diameter of the case, and the lateral cross-sectional area of the core and case is at most ten times the lateral cross-sectional area of the core;
b. inserting the core into the case to form an elongate composite filament in which the case surrounds the core;
c. cold-working the composite filament to reduce the lateral cross-sectional area of the composite filament by at least 15%, whereby the composite filament has a selected diameter less than an initial outside diameter of composite filament before cold-working;
d. annealing the composite filament after cold-working, to substantially remove strain hardening and other stresses induced by the cold-working step;
e. mechanically forming the annealed composite filament into a predetermined shape; and
f. after the cold-working and annealing steps, and while maintaining the composite filament in the predetermined shape, age hardening the composite filament.
In one preferred version of the process, the radiopaque material has a linear attenuation coefficient, at 100 KeV, of at least 25 cmxe2x88x921. The radiopaque material forms the core, and is at least as ductile as the case. The outside diameter of the composite filament, before cold-working, preferably is at most about six millimeters (about 0.25 inches). The cold-working step can include drawing the composite filament serially through several dies, with each die plastically deforming the composite filament to reduce the outside diameter. Whenever a stage including one or more cold-working dies has reduced the cross-sectional area by at least 25%, an annealing step should be performed before any further cold-working.
During each annealing step, the composite filament is heated to a temperature in the range of about 1700-2306xc2x0 F. more preferrably 1950-2150xc2x0 for a period depending on the filament diameter, typically in the range of several seconds to several minutes. The core material and cladding (case) materials preferably are selected to have overlapping annealing temperature ranges, and similar coefficients of thermal expansion. The core and case materials further can be selectively matched as to their crystalline structure and metallurgical compatibility.
In an alternative version of the process, the initial outside diameter of the composite structure (billet) typically is at least fifty millimeters (about two inches) in diameter. Then, before cold-working, the composite filament is subjected to temperatures in the annealing range while the outside diameter is substantially reduced, either by swaging or by pulltrusion, in successive increments until the outside diameter is at most about 6 millimeters (0.25 inches). The resulting filament is processed as before, in alternative cold-working and annealing stages.
Further according to the process, the composite filament can be severed into a plurality of strands. Then, the strands are arranged in two oppositely directed sets of parallel helical windings about a cylindrical form, with the strands intertwined in a braided configuration to form multiple intersections. Then, while the strands are maintained in a predetermined uniform tension, they are heated to a temperature in the range of about 700-1200xc2x0 F., more preferably 900-1000xc2x0 F., for a time sufficient to age harden the helical windings.
The result of this process is a resilient, body implantable prosthesis. The prosthesis has a plurality of resilient strands, helically wound in two oppositely directed sets of spaced apart and parallel strands, interwoven with one another in a braided configuration. Each of the strands includes an elongate core and an elongate tubular case surrounding the core. A cross-sectional area of the core is at least ten percent of the cross-sectional area of the strand. The core is constructed of a first material having a linear attenuation coefficient of at least 25 cmxe2x88x921 at 100 KeV. The case is constructed of a resilient second material, less ductile than the first material.
More generally, the process can be employed to form a body compatible device comprising an elongate filament substantially uniform in lateral cross-section over its length and including an elongate cylindrical core and an elongate tubular case surrounding the core. One of the core and case is constructed of a first material having a yield strength (0.2% offset) of at least twice that of the second material. The other of the core and case is constructed of a second material being radiopaque and at least as ductile as the first material.
In a highly preferred version of the invention, the core is constructed of tantalum for radio-opacity, and the case is constructed of a cobalt-based alloy, e.g. as available under the brand names xe2x80x9cElgiloyxe2x80x9d, xe2x80x9cPhynoxxe2x80x9d and xe2x80x9cMP35Nxe2x80x9d. The xe2x80x9cElgiloyxe2x80x9d and xe2x80x9cPhynoxxe2x80x9d alloys contain cobalt, chromium, nickel, and molybdenum, along with iron. Either of these alloys is well matched with tantalum, in terms of overlapping annealing temperature ranges, coefficients of thermal expansion and crystalline structure. The tantalum core and alloy case can be contiguous with one another, with virtually no formation of intermetallics.
When otherwise compatible core and case materials present the risk of intermetallic formation, an intermediate layer, e.g. of tantalum, niobium, or platinum, can be formed between the core and the case to provide a barrier against intermetallic formation. Further, if the case itself is not sufficiently biocompatible, a biocompatible coating or film can surround the case. Tantalum, platinum, iridium titanium and their alloys, or stainless steels can be used for this purpose.
While disclosed herein in connection with a radially self-expanding stent, the composite filaments can be employed in constructing other implantable medical devices, e.g. vena cava filters, blood filters and thrombosis coils. Thus, in accordance with the present invention there is provided a resilient, body compatible prosthesis which, despite being sufficiently small for placement within blood vessels and similarly sized body cavities, has sufficient radio-opacity for fluoroscopic imaging based on the prosthesis materials themselves.